The invention relates to granular polycrystalline silicon and to production thereof.
Granular polycrystalline silicon, or granular polysilicon for short, is an alternative to the polysilicon produced in the Siemens process. While the polysilicon in the Siemens process is obtained as a cylindrical silicon rod which has to be comminuted prior to further use thereof in a time-consuming and costly manner to give what is called chip poly, and may again have to be purified, granular polysilicon has bulk material properties and can be used directly as a raw material, for example for single crystal production for the photovoltaics and electronics industry.
Granular polysilicon is produced in a fluidized bed reactor. This is accomplished by fluidization of silicon particles by means of a gas flow in a fluidized bed, the latter being heated to high temperatures by means of a heating apparatus. Addition of a silicon-containing reaction gas results in a pyrolysis reaction at the hot particle surface. This deposits elemental silicon on the silicon particles, and the individual particles grow in diameter. The regular removal of particles which have grown and addition of relatively small silicon particles as seed particles enable continuous operation of the process with all the associated advantages. The silicon-containing reactant gas used may comprise silicon-halogen compounds (e.g. chlorosilanes or bromosilanes), monosilane (SiH4) and mixtures of these gases with hydrogen. Such deposition processes and apparatuses for this purpose are known, for example, from US2008299291.
A factor of great significance for performance of granular polysilicon in its applications is a defined, constantly low halogen content.
Performance is understood to mean, for example, the avoidance of spattering effects in the course of crystal pulling.
Halogens such as chlorine constitute a main impurity in polysilicon production if halosilanes such as trichlorosilane, in which halogens are present in large amounts, are used as a precursor.
It is known that the chlorine content which is established in the polysilicon product depends on the process conditions in the fluidized bed reactor.
For instance, U.S. Pat. No. 5,077,028 already discloses that, with rising linear growth rate, a lower chlorine content in the product can be obtained. It is possible to produce granular polysilicon with a chlorine content of less than 20 ppmw at a crystal growth rate of greater than 0.4 μm/min. In order to ensure such a crystal growth rate, the temperature and residence time should be adjusted accordingly. A chlorine content of less than 20 ppmw ensures that no scattering effects occur in the course of crystal pulling.
U.S. Pat. No. 5,037,503 discloses the production of single silicon crystals using granular polysilicon having a chlorine content of less than 15 ppmw (deposition with trichlorosilane) or a hydrogen content of 7.5 ppmw (deposition with silane). In the case of use of such granular polysilicon, no spattering effects occur in the course of crystal pulling.
U.S. Pat. No. 6,007,869 claims granular polysilicon having a chlorine content of 6-47 ppmw. As a result of the low chlorine content, adverse effects are avoided in the course of CZ pulling, such as poor single crystal quality, spattering effects and formation of corrosive gases. For production of the granular polysilicon, a reaction gas temperature greater than 900° C. and a particle temperature greater than 1000° C. have to be selected.
In the prior art, online analysis is used for process control in various production processes.
DE102009043946 discloses a method for controlling a system for the production of polycrystalline silicon, wherein the system comprises at least one reactor with at least one inlet pipe and an outlet pipe for a mixture of gases, characterized by the following steps:                withdrawing specimens to be measured from the inlet pipe and the outlet pipe of each reactor;        feeding the withdrawn specimens to be measured to at least one gas-phase chromatograph via a pipe in each case;        obtaining controlling signals on the basis of the measurements with respect to the composition of the fed specimens to be measured obtained by the gas-phase chromatograph; and        adjusting a plurality of parameters of the at least one reactor on the basis of the obtained controlling signals by means of a control unit via actuators in such a way that the efficiency of the system automatically results in a production optimum.        
U.S. Pat. No. 4,148,931A discloses a process for depositing elemental silicon from a reactive gas comprised of a mixture of a halogen silane and hydrogen over heated carrier bodies (Siemens process), wherein the concentration of hydrogen halide in an exhaust gas is monitored and used to control the supply of the reactive gas so as to regulate the flow of reaction gas in such a manner that the concentration of said hydrogen halide in the exhaust gas remains chronologically substantially constant. This is done by using the content of hydrogen halide in the exhaust gas as a controlled variable for control of the manipulated variable of the input gas flow. The aim is to maintain a specific deposition rate remaining constant over the reaction process.
It has been found that the aforementioned processes lead to polysilicon which avoids spattering effects in the course of crystal pulling owing to the low chlorine content, but causes some stacking faults.
In addition, the bulk material properties and the fluidization properties of the particles, and much more significantly of the chip poly too, are unsatisfactory.
A further disadvantage occurs in the measurement of the fluidized bed temperature. Dust formation disrupts the temperature measurement, and it cannot be used as a controlled variable for control of the reactor for a stable product quality.
The problems described gave rise to the objective of the present invention.